Electron discharge device



July 29, 1941. c. J. DAVISSON ELECTRON DISCHARGE DEVICE Filed May 2,1939 h EA Al b.?

.2 S25 13% E C asundnr o1 JI ll H 8 k a mm m N Q\h\ A T TOPNE Y PatentedJuly 29,1941

, lJNlTE STTES ATENT OFFICE ELECTRON DISCHARGE DEVICE Application May 2,1939, Serial No. 271,295

4 Claims.

This invention relates to electron discharge devices and morespecifically to electron camera tubes for television.

In British Patent 369,832 to Zworykin, March 31, 1932, there isdisclosed an electron camera tube for television transmission comprisingan evacuated envelope having at one end thereof means for generating abeam of electrons and at the other end thereof a screen structurecomprising, in the simplest embodiment shown, a mesh screen, a metallicplate having a plurality of apertures therein, each of said aperturesbeing filled with an insulating insert pierced by a metallic plug memberhaving aglobular-shaped head, and a secondmesh screen which is coveredwith photoemissive material. Radiations from an object or field of vieware applied to the mesh screen which is covered with photoemissivematerial to cause the emission of electrons from the screen to themetallic plug members where charges are stored because of the capacitybetween each of the plugs and the metallic plate, the insulating insertsbeing used as dielectrics. The plugs are scanned through the first meshscreen to remove these charges and to set up an image current.

It is an object of this invention to provide an electron camera tube ofthe general type disclosed in the British patent but which by using ascreen structure which is much simpler to construct and which embodiesfewer parts, is a much more simple and satisfactory tube.

It is another object of this invention to provide an electron cameratube in which the capacity between a conducting pho-toemissive screenand a plurality of metallic plugs carried in an insulating plate isutilized for the storage of charges.

Other and ancillary objects and features of this invention will beapparent from the description below and from the appended claims.

In accordance with this invention there is provided, for example, atelevision transmitter tube comprising a cathode ray device on the endwall of which is a translucent conducting photoemissive surface.Parallel and very close to the photoemissive surface is a mosaic ofsmall metallic conducting buttons which are supported and insulated fromone another by a perforated plate of insulating material, such as glass,the con ducting elements extending through the plate from side to side.An electron gun is provided for generating a beam of electrons and theanode portion of said gun near the photoemissive surface is connected toground. The photoemissive surface is connected to ground through aresistance and a source of potential which maintains it at a potentialof 50 to: volts negative with respect to ground. The buttons of themosaic are similar and their ends have a coilicient of secondaryemission which is for electrons of the speed of those in the scanningbeam (from 1000 to 1500 volts) appreciably greater than 1, that is, forpractical purposes, not less than 1.5.

The operation of the electron camera tube of this invention is asfollows: With the transmitter operating, normally the dark potential ofthe mosaic is at or near earth potential. The photoplate on the end wallof the tube is 50 to 100 volts negative with respect to earth. Eachelement of the mosaic, that is, each metallic button, then has a certaindark charge which is positive and which is greater the greater thecapacity between the photo-plate and the mosaic,

1 and which is proportional to the negative potential applied to thephoto-plate, the minus 50 or minus 100 volt-s. The effect of thephotocurrent which fiows from the photo-plate to each of the plugelements during the scanning period is to discharge this elementarycondenser. The action of the scanning beam is thus to restore theelementary condenser to its fully charged condition. It is importantthat the photo-currents from even the most brightly illuminated parts ofthe plate shall not completely discharge the associated elementarycondenser in the time of one scanning period. The signal from an elementis proportional to the extent to which the elementary condenser isdischarged. If an elementary condenser were completely discharged in thescanning period for some intermediate plate brightness then it would becompletely discharged for all greater brightnesses and all brightnessesabove the critical brightness would give signals of the same strength;that is, all parts of the object above the critical brightness wouldappear in the picture as of the same brightness. The capacity betweenthe photoplate and the mosaic must be large enough so that none of theelementary condensers is ever completely discharged. If this conditionis not satisfied the high-lights of the image are flattened out. If thiscondition is satisfied the transmitter is operative and increasing thecapacity or lowering the plate voltage further has no effeet. If thiscapacity is large enough, it is not necessary that any metallic elementbe placed in capacitive relation to the metallic button of the mosaic,as is the case in the British patent described above. Thus it ispossible to make use of a greatly simplified target plate.

An important feature of the present invention has to do with the strongand definite field which is provided for transporting photoelectronsfrom the photo-plate to the elements of the mosaic, that is, the 50 to100-volt field between plate and mosaic. Due to the closeness of thephotoplate and the mosaic, preferably of the order of from one to tenthousandths of an inch, the voltage gradient between these two membersis very large and this field insures that every photoelectron emitted bythe plate is transferred to and stores on its proper element. A weakfeature of the well known iconoscope is that no positive means isprovided for carrying off the photo-emission from the element, thisobjectionable feature not being present in the instant invention. In thearrangement shown in the British patent (where the plugs serve toattract electrons from the photoemissive mesh screen) there are metallicelements between the various plug members, which metallic elements willattract certain of the electrons from the photoemissive screen and thusprevent these electrons from going to the plug members. In the presentinvention there are no metallic elements in the mosaic target other thanthe plug members so there is nothing to prevent the photoelectrons fromgoing directly across to the plug members.

The invention will be more readily understood from the followingdescription taken in connection with the accompanying drawing forming apart thereof, in which:

Fig. 1 shows schematically an electron camera tube and associatedcircuits in accordance with the invention;

Fig. 2 is a sectional View of a complete tube embodying the inventionshowing the mosaic target before it has been put into position closelyadjacent the photoemissive plate member on the wall of the tube; and

Fig. 3 is an end view of a portion of the tube shown in Fig. 2.

Referring more particularly to the drawing, Fig. 1 shows schematically acathode ray television transmitter in accordance with the invention. Thetransmitter preferably comprises an evacuated container enclosing anelectron gun for generating and focussing a beam of electrons, a targetassembly T, means for causing the beam to scan every elemental area inturn of a field on the target assembly T and a conducting translucentphotoemissive layer P upon which radiations from an object or field ofview are projected. The envelope surrounding the electron gun deflectingsystem and target assembly is, for simplicity, not shown in this figurebut is shown in Fig. 2.

The electron gun assembly comprises an equipotential cathode I9, heatedby a suitable heater, a shield electrode II, a first anode I2 and asecond anode comprising three coaxial cylinders I3, I4 and I5 all ofwhich are placed at the same potential, Which potential is preferablyground. The cathode i ll is placed at a potential from 1000 to 1500volts below ground by means of a source IS. The first anode I2 is placedat a potential about 600 volts positive with respect to the cathode bymeans of the source IT. The various elements in the electron gun systemcooperate in a manner well known to those skilled in electron optics toform a beam of small cross-section at the target T.

In order to cause the electron beam generated by the electron gunapparatus described above to scan every elemental area of the field ofView on the target T in turn, suitable deflecting plates such as, forexample, two pairs of electrostatic deflecting plates I8, I8 and I9, I9,the normals to whose surfaces are located at right angles to each other,are provided. To the deflecting plates I8, I8 are applied deflectingvoltages at line scanning frequency, (5760 cycles per second, forexample) and of saw-toothed wave form to produce the horizontaldeflection, while deflecting voltages of framing frequency (24 cyclesper second, for example) and of saw-toothed wave form are applied to thedeflecting plates I9, I9 to produce vertical deflection of the beam. Anyappropriate sweep circuit (not shown) may be used to generate thehorizontal and vertical deflection voltages, For example, reference maybe made to Patent 2,178,464, issued October 31, 1939, to M. W. Baldwin,Jr., which discloses suitable balanced sweep circuits for this purpose.Connections may be made from the balanced sweep circuits to the pairs ofdeflecting plates I8, l8 and I9, I9 by means of coupling condensers 20,2I and 22, 23, respectively, of about 1 microfarad capacity each.Coupling resistances 24 and 25 of the order of 20 megohms each arerespectively connected across the pairs of plates I8, I8 and I9, I9. Themid-points of the resistances 24 and 25 are connected to ground so thatthe average of the potentials of the deflecting plates does not deviatemore than slightly from the potential of the second anode elements l3,I4 and I5. This relationship is maintained to avoid changes in thesensitivity of the deflecting system, and the consequent distortion ofthe image which would otherwise result. For a fuller description of theadvantages of balanced sweep circuits for use with cathode raytelevision tubes, reference may be made to the above-mentioned Baldwinpatent and also to Patent 2,209,199, issued July 23, 1940, to FrankGray.

The target T comprises an insulating plate 26 comprising a thin film ofglass about .002 inch thick supported by a thicker border of glass (notshown), the thin center portion having therein a multiplicity of smallholes regularly spaced and of uniform size. These holes are filled withlocked-in metallic buttons 21. The mosaic plate is preferably preparedby coating one surface of a glass plate with a plurality of wax dotsthrough a mesh screen, removing the screen, melting the wax so that onlysmall spaces of the plate between the dots are left uncovered, dippingthe plate in an etching solution until the acid almost etches through tothe side which is not covered by the wax, etching from the other sideuntil the acid etches through to the apertures, washing the plate, andthen depositing metallic plugs in the apertures from a suitable metallicplating solution. Large heads are then put on the plugs to cause them tobe locked in place. For a fuller description of targets of the typedisclosed herein and for a method of making them, reference may be madeto Patent 2,217,334, issued October 8, 1940, to B. A. Diggory and G. K.Teal.

Closely adjacent the surface of the target T remote from the electrongun is a conducting photoemissive surface (photo-plate) P. This surfaceis connected through a signal resistance R and at least a portion ofsource 28 to ground. The photoemissive layer P is preferably maintainedat a potential of about 50 or volts negative with respect to ground.

The buttons 21 of the mosaic target T are similar and their ends have acoemcient of secondary emission which is, for electrons of the speed ofthose in the scanning beam, appreciably greater than 1-not less, forexample, than 1.5.

The currents Within the tube are as follows: With the photo-plate P dark(that is, with no radiations being projected upon the photo-plate P fromthe object through the lens L) and the scanning beam sweeping the mosaicT, the system attains a dynamic equilibrium in which the net current tothe mosaic will be zero. The current of electrons flowing to the mosaicT from the scanning beam is balanced by an equal current of secondaryelectrons flowing from the mosaic to the secondary anode member [5. Theequilibrium results from an adjustment of potentials which reduces thenumber of escaping secondaries (those passing from the mosaic T to theanode member it) to an average of 1 per primary electron. The nature ofthis and similar equilibria will be considered more fully below.

If the beam current is switched off and the photo-plate P uniformlyilluminated, photoelectrons will flow from the plate P to the mosaictarget T. This continues until the potential of the target T fallssufiiciently far below that of the plate P to reduce the photo-currentto zero.

Considering now that the scanning beam is switched on but at anextremely low current, so low that the total secondary emission from thesurfaces of the metallic buttons 21 near the scanning beam is smallcompared to the total photo-emission from P, the mean potential of themosaic T will rise until a photo-current equal in magnitude to the totalsecondary emission minus the beam current flows from the photoplate P tothe mosaic target T. An equilibrium is established in which the beamcurrent plus the photo-current (a part only of the total photoemission)is equal to the total secondary emission. The potential of the mosaicdiifers only slightly from that of the photo-plate P.

If the beam current is increased, the total secondary emission alsoincreases and the mean potential of the mosaic rises still more, butstill only silghtly to maintain the type of equilibrium just described.This continues with continuously rising beam current until the totalsecondary emission is equal to the beam current plus the totalphoto-emission. When this condition is reached the equilibrium isunstable. The system is in equilibrium for any mean potential of themosaic which is sufficiently above that of the photo-plate P to saturatethe photo-emission and sufficiently below that of the secondary anodemember I to allow all secondary electrons to escape.

For still greater beam current, equilibrium is attained with the meanpotential of the mosaic near that of the secondary anode member It. Thetotal secondary emission is greater than the beam current plus the totalphoto-emission. The potential of the mosaic target T rises until thecurrent of escaping secondary electrons is just equal to the sum ofthese two currents flowing into the mosaic. It is this latter type ofequilibrium only which is of interestthat which results when the totalsecondary emission is greater than the beam current plus thephoto-current. It will be assumed in what follows that this condition isalways satisfied.

It will now be considered what it is that happens when the illuminationof the photo-plate P is constant in time but non-uniform indistribution-that is, when some portions are-illuminated more brightlythan others by radiations froman object O to be televised. The currentof photoelectrons flowing from the plate P to the mosaic T is constant;so also is the beam current. The current of secondary electrons flowingfrom the mosaic T to the second anode member I5 is, however, notconstant. As regards a single element of the mosaic T the equilibrium isone in which the element receives photoelectrons at a constant rate fromthe portion of the photoplate P which it faces and then once eachscanning period, while the beam is passing over it, receives electronsat a high rate from the beam and at the same time loses secondaryelectrons to the second anode member l5 at an even higher rate. Theelement receives, for example, n photo-electrons during the scanningperiod and then, while the beam sweeps it, receives N electrons from thebeam and loses (N-i-n) to the secondary anode member l5. The current tothe secondary anode member I5 is greater when the beam is sweepingelements which face brightly illuminated portions of the photo-plate Pthan when it is sweeping elements which face less brightly illuminatedareas. In fact, the secondary current minus the beam current is at everyinstant proportional to the brightness of that portion of thephoto-plate P opposed to the elements being swept by the beam. Thesecondary current is not at every instant equal to the beam current plusthe total photo-current. This equality holds only for the average valueof the secondary current taken over the complete scanning period.

There is one other current which should be considered. This is thecurrent of electrons which flows into the photo-plate P through theresistance R and which serves to compensate or balance the flow ofphotoelectrons from the photoplate P to the mosaic target T. It isconceivable that this current is constant like the total photocurrent orthat it is variable like the secondary current. As a matter of fact itis variable like the secondary circuit. It is not strictly equal atevery instant to the secondary current minus the beam current, but whencertain circuit conditions are satisfied, it duplicates this currentdifference with high, though not perfect, fidelity.

It follows that when these conditions are satisfled the potential acrossthe resistance R is closely proportional at every instant to thebrightness of the photo-plate P opposite the mosaic element upon whichthe scanning beam is playing. This voltage constitutes the televisionsignal.

Referring now to Fig. 2, a cathode ray device 39 is shown in which thepresent invention is used. The device 30 comprises a large tube 31 and asmaller tube 32. The smaller tube 32 includes the cathode It, shield H,first anode l2, the first and second members l3 and l i of the secondanode member, and the deflecting plates l8, l8 and I9, I 9. The largertube 3! contains cylindrical members 33 and 34, forming together thethird member of the second anode member, the mosaic member T and thephoto-plate P. It is to be understood, of course, that any othersuitable electron gun system may be used instead of the one specificallydescribed above.

The cylinder 33 is preferably supported within the envelope 3| by meansof support and contact wires 35 and 36, while the inner cylinder 34 isslidingly supported by means of sliding contact members 3! which serveboth to hold the cylinder 34 in position with respect to thelongitudinal axis of thetube and also to make contact-with-the'cylindrical member 33. Thus electrically the cylinders 33 are onemember.

The mosaic target T is supported from the cylinder 34 by means of smallsprings 38 (see Fig. 3). Mica spacers 39 are also used under the springs38 to space the mosaic target T the proper distance from thephotoelectric plate P, when the target T is moved up to its operatingposition (see the dotted line position, Fig. 2). In practice, thisdistance should be from one to ten thousandths of an inch.

The photo-plate P comprises a glass plate 39 which has four glass beads49 around the periphery thereof, from which wires 4| extend to serve assupporting members for the plate P. The glass plate 39 is thus mountedcontiguous to a glass plate 42 which serves as the end wall of thecathode ray 30. The glass plate 39 is coated with platinum and thensilver, the silver being oxidized and coated with caesium to form aphotoemissive layer on the plate 39. Thus a conducting photoemissivesurface 43 is formed on the side of the photo-plate P adjacent themosaic layer T. A contact 44 is made to the photoelectric surface 43 bymeans of one of the wires 4| extending through the seal 45.

The spacing between the photo-plate P and the mosaic target T is one ofthe factors determining the capacity between these elements. Thiscapacity must be sufiiciently large to insure that none of theelementary condensers of the mosaic is completely discharged (by theemission of photoelectrons from the photo-plate P) during the scanningperiodeven those opposite the most brightly illuminated plate areas.This capacity has, however, another function for which it is desirablethat the capacity be as large as possible, or at any rate quite high.The signal current which flows into the, plate P through the signalresistance R is not precisely or identically that due to the chargeswhich flow onto the parts of the plate P opposite the elements of themosaic T when the elements are discharged. These induced plate chargeswhich account for the signal current are always somewhat less than theelement charges, but the discrepancy between them is less, the greaterthe plate to mosaic capacity. This capacity should be great enough tomake these charges substantially equal, or nearly so, if the signals areto be as strong as possible. It is desinable also that the plate tomosaic separation be as small as possible to insure that the emissionfrom the part of the plate opposite the given element. reaches thiselement and not to any appreciable extent the neighboring elements. Thisrequirement also leads incidentally to a high capacity from plate tomosaic.

In assembling the tube the cap 42 is sealed on the end of the tube 3|and the cylinder 34 is shaken down so that the spacers 39 adjacent themosaic target T rest against the photo-plate P. Thus there is a spacingof only a small fraction of an inch, as stated above a few thousandthsof an inch, between the end of the plugs 21 and the photoemissivesurface 43. The tube 32 is then sealed into the larger tube 3|. Theglass plate 3%! has a coating of platinum previously applied thereto anda coating of silver placed thereon. Oxygen is admitted to the tube and aspark coil placed close to the tube until silver oxide is formed. Theoxide is then photosensitized by firing a caesium pill in the side tube46. Due to the presence of the platinum layer the photoemissive surface43 is conducting and connected by means of a connecting wire 44 to theresist ance R which is connected to the input circuit of a suitableamplifier by means of a coupling condenser 41 (see Fig. 1), for example.

When radiations from an object O are projected upon the photoemissivesurface 43 by means of a suitable lens system L and the plugs 21 areeach scanned in turn by means of the electron beam generated by theelectron gun, a television signal current appears in the resistance Rwhich is amplified by suitable means and which may be transmitted overwire or carrier channel to a television receiving station.

What is claimed is:

1. A television camera tube comprising means for generating a beam ofelectrons, a photoemissive screen, a continuous plate of insulatingmaterial having a multiplicity of metallic conductors therethrough, eachof said conductors being separated from adjacent conductors solely byinsulating material, said plate being placed between said screen andsaid beam generating means, means for subjecting said screen toradiations to cause emission of electrons from said screen to the endsof the conductors in said plate near said screen, and means for causingsaid beam of electrons to repeatedly scan the ends of said metallicconductors remote from said screen, said photoemissive screen and saidinsulating plate being substantially parallel to each other andseparated by a distance of from one to ten thousandths of an inch.

2. A television camera device comprising a gastight container, means insaid container for generating a beam of electrons, a photoemissivescreen remote from said beam generating means a plate of insulatingmaterial having a multiplicity of elemental metallic plug conductorstherethrough, each of said conductors being separated from adjacentconductors solely by insulating material, said plate being placedbetween said screen and said beam generating means, means includingapparatus for causing said beam to repeatedly scan the ends of theconductors in said insulating plate remote from said screen to chargeevery plug to a positive equilibrium potential with respect to thepotential of said screen, and means for applying radiations from anobject or field of view to said photoemissive screen to cause theemission of electrons from the various elemental areas thereof to theends of the metallic conductors in the insulating plate near said screento partially remove the charges on said plugs, the degree of removal ofthe charge on any plug being proportional to the light-tone value of thecorresponding elemental area of the object or field of view, said screenand said insulating plate being separated by a distance of from one toten thousandths of an inch.

3. A television camera device comprising a gastight container, means insaid container for generating a beam of electrons, comprising a cathodeand a plurality of anodes, a conducting photoemissive screen remote fromsaid beam generating means, a plate of insulating material having amultiplicity of elemental metallic plug con-ductors therethrough, eachof said conductors being separated from adjacent conductors solely byinsulating material, said plates being placed between said screen andsaid beam generating means, means for placing said photoemissive screenat a potential of from 50 to volts negative with respect to the anode ofsaid beam generating means nearer said plate of insulating material,

means including apparatus for causing said beam to scan repeatedly theends of the conductors in said insulating plate remote from said screento charge every plug to a positive equilibrium potential with respect tothe potential of said screen, means for applying radiations from anobject or field of View to said photoemissive screen to cause theemission of electrons from the various elemental areas thereof to theends of the metallic conductors in the insulating plate near said screento partially remove the charges on said plug, the degree of removal ofthe charges on any plug being proportional to the light-tone value ofthe corresponding elemental area of the object or field of View, andspacers for separating said screen and said insulating plate by from oneto ten thousandths of an inch.

4. A television camera tube comprising means for generating a beam ofrelatively high velocity electrons, a target for said beam comprising acontinuous plate of insulating material having a multiplicity ofmetallic conductors therethrough, each of said conductors being free toswing in potential and each being separated from adjacent conductorssolely by insulating material, a photoemissive screen, all portions ofwhich are at the same potential, positioned closely adjacent said targetand facing the side thereof remote from said beam generating means,means for subjecting said screen to radiations from an object or fieldof view to cause the emission of photoelectrons from the variouselemental portions thereof: means for providing an accelerating fieldfor the photoelectrons emitted from said photoemissive screen, said lastmentioned means including means for causing said beam of relatively highvelocity electrons to scan in succession the end of each of the metallicconductors in said target remote from said screen to cause the emissionof secondary electrons therefrom to an extent which makes the potential'of each of said conductors swingin a positive direction, to anequilibrium potential, with respect to the potential of thephotoemissive screen, said photoelectrons causing the lowering of thepotential across and thus the partial discharge of each of the elementalcapacities between a metallic conductor and the photoemissive screen inan amount proportional to the light-tone value of the correspondingelemental area of the object, and means including a conductive elementpassing through the envelope of said tube and which is connected to saidphotoemissive screen for passing an image current formed when the beamof electrons returns the conductors to their equilibrium potential, saidscreen and said plate being placed so close together that the capacitytherebetween is so great that none of the elemental condensers is evercompletely discharged by the passage of photoelectrons from said screento said conductors, whereby said conductors are at all times at apositive potential With respect to said screen.

CLINTON J. DAVISSON.

